Solid-state photodetector with variable spectral response

ABSTRACT

A solid-state photodetector with variable spectral response that can produce a narrow or wide response spectrum of incident light. Some embodiments include a solid-state device structure that includes a first photodiode and a second photodiode that share a common anode region. Bias voltages applied to the first photodiode and/or the second photodiode may be used to control the thicknesses of depletion regions of the photodiodes and/or a common anode region to vary the spectral response of the photodetector. Thickness of the depletion regions and/or the common anode region may be controlled based on resistance between multiple contacts of the common anode region and/or capacitance of the depletion regions. Embodiments include control circuits and methods for determining spectral characteristics of incident light using the variable spectral response photodetector.

CROSS REFERENCE

The present application claims priority under 35 U.S.C. §119 to U.S.Provisional Patent Application No. 61/673,079, entitled “SOLID-STATEPHOTODETECTOR WITH VARIABLE SPECTRAL RESPONSE,” filed Jul. 18, 2012, theentire disclosure of which is incorporated herein by reference for allpurposes.

BACKGROUND

1. Field

The present disclosure relates to light sensors in general and, inparticular, to a solid-state photodetector with voltage variablespectral response.

2. Background

Light sensors or photodetectors have many applications in a variety offields from scientific instruments to consumer electronics. Lightsensors may be used to measure properties of light at particularwavelengths of interest or over a range of wavelengths. For example, acolor analyzer may determine the color properties of incident light.Color analyzers may be used to compare color levels of materials orcolor reference of displays such as computer monitors or televisions.

Spectrometers measure the properties of light over a range ofwavelengths of light. Spectrometers may be used for spectroscopy of alight source to determine spectral properties of the light source.Spectrometers may also be used in spectroscopy (e.g., Ramanspectroscopy, infrared spectroscopy, etc.) to determine chemical orphysical properties of an illuminated sample.

Many current light sensors use photodiodes that are sensitive to lightin a range of frequencies. Particular light components (i.e., colors,frequencies, or ranges of frequencies of light) may be detected throughthe use of color filters or diffraction gratings. For example, a coloranalyzer may have three photodiodes with red, green, and blue colorfilters to isolate components of incident light. To sense lightintensity across a range of wavelengths, spectrometers typically useoptical prisms or diffraction gratings to separate light into componentwavelengths and an array of photodiodes that detect the refracted ordiffracted light to measure the spectral components.

SUMMARY

Various embodiments described herein are directed to a solid-statephotodetector with variable spectral response. Some embodiments includea photodiode with a voltage variable depletion region thickness that canproduce a narrow or a wide response spectrum anywhere in the long waveUV to shortwave infrared band. Some embodiments include a solid-statedevice structure that forms a first photodiode having a first depletionregion proximate to a light acceptance surface of the photodetector anda second photodiode having a second depletion region, where the firstdepletion region is between the light acceptance surface and the seconddepletion region. The first depletion region and the second depletionregion may be separated by a common anode region of the first and secondphotodiodes.

Bias voltages of the first photodiode and the second photodiode maydetermine the thicknesses of the depletion regions and the common anoderegion to vary the spectral response of the first photodiode based onabsorption depth of incident light. In embodiments, thickness of thefirst depletion region of the first photodiode and the common anoderegion are controlled based on resistance measurements between multiplecontacts of the common anode region. In embodiments, thickness of thedepletion region of the first photodiode and the common anode region arecontrolled based on capacitance of the first photodiode depletion regionand/or the second photodiode depletion region.

Measured photocurrent of the first photodiode may be processed atmultiple voltage bias settings corresponding to various thicknesses ofthe first depletion region of the first photodiode to determinespectrally dependent photocurrent measurements for various ranges ofincident light. Measured photocurrent of the second photodiode may beused to adjust measured photocurrent of the first photodiode to producea desired response. The variable spectral response photodetector may beused in light sensing applications including human eye response sensors,color analyzers, spectrometers, and the like.

Some embodiments include an apparatus for sensing incident light thatincludes a light detector that receives the incident light at a lightacceptance surface. The light detector may include a first photodiodethat absorbs a first portion of the incident light in a first depletionregion and generates a photocurrent responsive to the absorbed firstportion of incident light. The light detector may include a secondphotodiode that absorbs a second portion of the incident light in asecond depletion region. The first depletion region of the firstphotodiode may be between the second depletion region of the secondphotodiode and the light acceptance surface. The apparatus may include adetector driver module coupled with the light detector that isconfigured to apply a first bias voltage to the first photodiode, wherea thickness of the first depletion region is controlled at least in partbased on the first bias voltage, apply a second bias voltage differentthan the first bias voltage to the first photodiode, where the thicknessof the first depletion region is controlled at least in part based onthe second bias voltage, measure the photocurrent of the firstphotodiode at each of the first and second bias voltages to obtain a atleast two photocurrent measurements, and determine a spectral componentof the incident light based at least in part on the at least twophotocurrent measurements.

In some embodiments the detector driver module may be configured toapply a third bias voltage to the second photodiode while applying thefirst bias voltage to the first photodiode, the thickness of the seconddepletion region controlled at least in part based on the third biasvoltage, measure the photocurrent of the first photodiode at the thirdbias voltage, apply a fourth bias voltage to the second photodiode whileapplying the second bias voltage to the first photodiode, the fourthbias voltage different from the third bias voltage, the thickness of thesecond depletion region controlled at least in part based on the fourthbias voltage; and determine the spectral component of the incident lightbased at least in part on the photocurrent measurements at the third andfourth bias voltages. The third and fourth bias voltages may be selectedsuch that a thickness of a common anode region between the firstdepletion region and the second depletion region is substantially thesame when the first and second bias voltages are applied. The thicknessof the common anode region may be controlled at least in part responsiveto a resistance of the common anode region. The detector driver modulemay be configured to control the thickness of the first depletion regionat each of the first and second bias voltages based at least in part ona capacitance of the first depletion region. The detector driver modulemay be configured to measure a photocurrent of the second photodioderesponsive to the absorbed second portion of the incident light at oneor more of the first and second bias voltages to obtain one or morebackgate photocurrent measurements and determine a second spectralcomponent of the incident light based at least in part on the at leasttwo photocurrent measurements and the one or more backgate photocurrentmeasurements.

In some embodiments, the detector driver module includes a first voltagecontrol module coupled with the first photodiode, a first currentmeasurement module coupled with the first photodiode, and a processormodule coupled with the first voltage control module and the firstcurrent measurement module, the processor module configured to determinethe spectral component of the incident light based at least in part onthe at least two photocurrent measurements via the first currentmeasurement module and light absorption depth information. The detectordriver module may include a second voltage control module coupled withthe second photodiode and a second current measurement module coupledwith the second photodiode, where the processor module is furthercoupled with the second voltage control module and the second currentmeasurement module, and where the processor module may be furtherconfigured to determine the spectral component of the incident lightbased at least in part on photocurrent measurements via the secondcurrent measurement module at each of the first and second biasvoltages.

Some embodiments include a method for sensing incident light received ata light acceptance surface that may include applying a first biasvoltage to a first photodiode that absorbs a first portion of theincident light in a first depletion region between the light acceptancesurface and a second depletion region of a second photodiode, the firstphotodiode generating a photocurrent responsive to the absorbed firstportion of incident light, a thickness of the first depletion regioncontrolled at least in part based on the first bias voltage, measuringthe photocurrent of the first photodiode at the first bias voltage toobtain a first photocurrent measurement, applying a second bias voltageto the first photodiode, the thickness of the first depletion regioncontrolled at least in part based on the second bias voltage, measuringthe photocurrent of the first photodiode at the second bias voltage toobtain a second photocurrent measurement, and determining a spectralcomponent of the incident light based at least in part on the firstphotocurrent measurement and the second photocurrent measurement.

In some embodiments, the method includes applying a third bias voltageto the second photodiode while applying the first bias voltage to thefirst photodiode, a thickness of the second depletion region controlledat least in part based on the third bias voltage, measuring thephotocurrent of the first photodiode at the third bias voltage to obtaina third photocurrent measurement, applying a fourth bias voltage to thesecond photodiode while applying the second bias voltage to the firstphotodiode, the fourth bias voltage different from the third biasvoltage, the thickness of the second depletion region controlled atleast in part based on the fourth bias voltage, and determining thespectral component of the incident light further based at least in parton the third photocurrent measurement and the fourth photocurrentmeasurement. Applying the third and fourth bias voltages may includecontrolling a thickness of a common anode region between the firstdepletion region and the second depletion region to be substantiallyequal when the first and second bias voltages are applied. The thicknessof the common anode region may be controlled at least in part responsiveto a resistance of the common anode region. Applying the first andsecond bias voltages may include controlling the thickness of the firstdepletion region at each of the first and second bias voltages based atleast in part on a capacitance of the first depletion region.

In some embodiments, the method includes measuring a backgatephotocurrent responsive to a second portion of the incident lightabsorbed in the second depletion region of the second photodiode at oneor more of said first and second bias voltages and determining a secondspectral component of the incident light based at least in part on atleast one of the first and second photocurrent measurements and the oneor more backgate photocurrent measurements. Determining the secondspectral component may include subtracting a portion of the one or morebackgate photocurrent measurements from the at least one of the firstand second photocurrent measurements, the portion of the one or morebackgate photocurrent measurements subtracted from the at least one ofthe first and second photocurrent measurements based at least in part onthe first bias voltage.

In some embodiments, the method includes measuring photocurrent at afirst plurality of bias points. The measurements at the first pluralityof bias points may be made by iteratively stepping the thickness of thefirst depletion region by a predetermined step thickness by, at least inpart, modifying the second bias voltage and measuring the photocurrentof the first photodiode. The method may include determining an amount ofreceived light at a second plurality of wavelength ranges by solving amatrix calculation based at least in part on the measured photocurrentat the first plurality of bias points and light absorption depthinformation. The number of photocurrent measurements measured at thefirst plurality of bias points may be greater than the number ofwavelength ranges of the second plurality of wavelength regions. Themethod may include adjusting the matrix calculation to adjust acalculated spectral response based at least in part on a metric of themeasured photocurrent at one or more bias points. The metric of themeasured photocurrent may include a combined incident light level.

Some embodiments include a photodetector that includes a firstphotodiode that absorbs a first portion of incident light in a firstdepletion region, the first photodiode generating a photocurrentresponsive to the absorbed first portion of incident light and a secondphotodiode that absorbs a second portion of the incident light in asecond depletion region, the absorbed second portion including a portionof the incident light not including the first portion of the incidentlight. Spectral response of the photocurrent may be controlled based atleast in part on a first bias voltage applied between a first cathodecontact coupled with a first cathode region of the first photodiode anda common anode contact coupled with a common anode region of the firstphotodiode and the second photodiode. The thickness of the firstdepletion region may be controlled at least in part by the first biasvoltage and a second bias voltage applied between the common anodecontact and a second cathode contact coupled with a second cathoderegion of the second photodiode. The common anode contact may include afirst common anode contact coupled with the common anode region and asecond common anode contact coupled with the common anode region,wherein a resistance between the first common anode contact and thesecond common anode contact depends at least in part on a thickness ofthe common anode region. The photodetector may include a semiconductorsubstrate layer comprising the first depletion region and a transparentgate electrode in between a light reception portion of the photodetectorthat receives the incident light and the first depletion region, whereinthe transparent gate electrode is biased to form the first cathoderegion of the first photodiode between the first depletion region andthe transparent gate electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the following drawings. In theappended figures, similar components or features may have the samereference label. Further, various components of the same type may bedistinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label. The drawingfigures are not necessarily drawn to scale and certain figures may beshown in exaggerated or generalized form in the interest of clarity andconciseness.

FIG. 1A illustrates a cross-section of a solid-state photodetector withvariable spectral response in accordance with various embodiments;

FIG. 1B shows a top or plan view of a solid-state photodetector withvariable spectral response in accordance with various embodiments;

FIG. 2A illustrates a first control state for a light detector deviceemploying a variable spectral response photodetector in accordance withvarious embodiments;

FIG. 2B illustrates a second control state for a light detector deviceemploying a variable spectral response photodetector in accordance withvarious embodiments;

FIG. 2C illustrates a third control state for a light detector deviceemploying a variable spectral response photodetector in accordance withvarious embodiments;

FIG. 3 illustrates example voltage settings for top and bottomphotodiodes of a variable spectral response photodetector in accordancewith various embodiments;

FIG. 4A illustrates a cross-section of a solid-state photodetector withvariable spectral response in accordance with various embodiments;

FIG. 4B shows a top or plan view of a solid-state photodetector withvariable spectral response in accordance with various embodiments;

FIG. 5A illustrates a cross section of a photodetector employing a MOSphotodiode cathode structure in accordance with various embodiments;

FIG. 5B illustrates a cross section of a photodetector employing a MOSphotodiode cathode structure in accordance with various embodiments;

FIG. 5C illustrates a plan view of a photodetector employing a MOSphotodiode cathode structure in accordance with various embodiments;

FIG. 6A illustrates example electron photocurrent response curves of afirst photodiode of a variable spectral response photodetector atvarious depletion region thicknesses, according to various embodiments;

FIG. 6B illustrates example backgate photocurrent response curves of avariable spectral response photodetector at various depletion regionthicknesses, according to various embodiments;

FIG. 7 illustrates an example of using a variable spectral responsephotodetector to sense light intensity at different wavelength regions;

FIG. 8A illustrates a cross section of a variable spectral responsephotodetector in accordance with various embodiments;

FIG. 8B illustrates a cross section of a variable spectral responsephotodetector employing an implant region to limit cathode surfaceleakage current in accordance with various embodiments;

FIG. 8C illustrates a cross section of a variable spectral responsephotodetector employing an edge gate to limit cathode surface leakagecurrent in accordance with various embodiments;

FIG. 9 illustrates a light detector apparatus, according to variousembodiments;

FIG. 10 illustrates a detector driver module for use in a light detectorapparatus or system, according to various embodiments;

FIG. 11 illustrates a light detector apparatus employing a variablespectral response photodetector, according to various embodiments;

FIG. 12 illustrates a processor block in accordance with variousembodiments;

FIG. 13 illustrates a color analyzer system employing a variablespectral response photodetector, according to various embodiments;

FIG. 14 illustrates a spectrometer system employing a variable spectralresponse photodetector, according to various embodiments;

FIG. 15 illustrates a flow diagram of a method for sensing incidentlight received at a light acceptance surface in accordance with variousembodiments; and

FIG. 16 illustrates a flow diagram of a method for sensing incidentlight received at a light acceptance surface in accordance with variousembodiments.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present disclosure are directed to a solid-statephotodetector with variable spectral response. Some embodiments includea photodiode with a voltage variable photocurrent collection boundarythat can produce a narrow or a wide response spectrum anywhere in thelong wave UV to shortwave infrared band. Some embodiments include asolid-state device structure that forms a first photodiode having afirst depletion region proximate to a light acceptance surface of thephotodetector and a second photodiode having a second depletion regionopposite of a common anode region from the light acceptance surface.

Bias voltages of the first photodiode and the second photodiode maydetermine the thicknesses of the depletion regions and the common anoderegion to vary the spectral response of the first photodiode based onabsorption depth of incident light. In embodiments, the thickness of thecommon anode region may be controlled to be relatively thin toaccurately control the depth of the photocurrent collection boundarybetween the first photodiode depletion region and the second photodiodedepletion region. In embodiments, thicknesses of the first photodiodedepletion region and the second photodiode depletion region arecontrolled based on resistance measurements between multiple contacts ofthe common anode region. In embodiments, thicknesses of the firstphotodiode depletion region and the second photodiode depletion regionare controlled based on capacitance of the first photodiode depletionregion and/or the second photodiode depletion region.

Measured photocurrent of the first photodiode may be processed atmultiple voltage bias settings corresponding to various thicknesses ofthe first depletion region of the first photodiode to determinespectrally dependent photocurrent measurements for various ranges ofincident light. Measured photocurrent of the second photodiode may beused to adjust measured photocurrent of the first photodiode to producea desired response. The variable spectral response photodetector may beused in light sensing applications including human eye response sensors,color analyzers, spectrometers, and the like.

This description provides examples, and is not intended to limit thescope, applicability or configuration of the invention. Rather, theensuing description will provide those skilled in the art with anenabling description for implementing embodiments of the invention.Various changes may be made in the function and arrangement of elements.

Thus, various embodiments may omit, substitute, or add variousprocedures or components as appropriate. For instance, it should beappreciated that the methods may be performed in an order different thanthat described, and that various steps may be added, omitted orcombined. Also, aspects and elements described with respect to certainembodiments may be combined in various other embodiments. It should alsobe appreciated that the following systems, methods, devices, andsoftware may individually or collectively be components of a largersystem, wherein other procedures may take precedence over or otherwisemodify their application.

FIG. 1A illustrates a cross-section of a solid-state photodetector 100-awith variable spectral response in accordance with various embodiments.In embodiments, variable spectral response photodetector 100-a includesvarious layers and/or regions doped to have the described electricalproperties including excess of negative charge carriers (N-type) orpositive charge carriers (P-type), as is known in the art. Variablespectral response photodetector 100-a may have a two-layer structuresuch as the structure illustrated in FIG. 1A including a P−substrate 110and P−epitaxial layer 120. In the two-layer structure illustrated inFIG. 1A, P−substrate 110 may include N+ buried implant layer 156. Thestructure of variable spectral response photodetector 100-a may furtherinclude N-well 154, N+ implant region 152, P+ implant regions 142 and/or144, and N+ implant region 132 in P− epitaxial layer 120.

For the convenience of description in the figures, a z-axis may bedefined as an axis orthogonal to substrate layers of the photodetector,while an x-axis and y-axis may define a plane parallel to a surface ofsubstrate layers of the photodetector. Also, for description purposes, anegative direction and a positive direction along the x-axis may bereferred to as a left direction and a right direction, respectively, anegative direction and a positive direction along the y-axis may bereferred to as a front direction and a rear direction, respectively, anda negative direction and a positive direction along the z-axis may bereferred to as a lower direction and an upper direction, respectively.In the ensuing description, various elements may be described by theirrelative positions based on the defined axes in the figures and theseidentifiers should not be construed to define absolute positionalrequirements of the photodetector or the various elements.

Generally, the various regions of variable spectral responsephotodetector 100-a may form a top photodiode 135 and a bottom orbackgate photodiode 155 that share a common anode region 140. The topphotodiode 135 may be electrically contacted through P+ implant regions142 and/or 144 (coupled to common anode region 140) and N+ implantregion 132 (cathode). The bottom photodiode 155 may be electricallycontacted through P+ implant regions 142 and/or 144 (coupled to commonanode region 140) and N+ implant region 152 (coupled to N+ buriedimplant layer 156 through N-Well 154). Coupling of electrical circuitsto the various implant regions may be accomplished through the use ofknown semiconductor processing steps (contacts, interconnect, wirebonding, etc.). Reference to a contact associated with one of thevarious implant regions assumes such electrical coupling as is known inthe art.

Generally, the ensuing description uses the example of a two-layerstructure with P− substrate 110 and P−epitaxial layer 120, however,other single-layer or multi-layer configurations, layer growth,deposition, or implant techniques may be used to form a variablespectral response photodetector according to various embodiments. Forexample, a variable spectral response photodetector may be constructedusing a single-layer or multi-layer configuration with N-typesemiconductor layers. The constituent semiconductor materials may be anyof various suitable semiconductor materials for photodetectorapplications such as germanium, gallium arsenide, indium galliumarsenide, indium phosphide, sapphire, and the like.

When a reverse bias voltage is applied to top photodiode 135, depletionregion 130 may form between the N+ implant or cathode region 132 and thecommon anode region 140. When a reverse bias voltage is applied tobottom photodiode 155, depletion region 150 may form between the commonanode region 140 and the N+ buried implant layer or backgate cathoderegion 156. The depth and thickness of the depletion region 130, thedepletion region 150, and the common anode region 140 may vary based onthe bias voltages applied to the top photodiode 135 and the bottomphotodiode 155.

Generally, a variable spectral response photodetector 100 may bearranged to accept incident light from a light source or object. In theembodiment illustrated in FIG. 1A, incident light 160 is received by thevariable spectral response photodetector 100-a at the silicon surface122. Variable spectral response photodetector 100 may have a metal layer180 that shields regions other than a portion of the semiconductorsurface 122 that is intended to receive the incident light 160, whichadvantageously may prevent edge complications from regions bordering topphotodiode 135 and bottom photodiode 155. As the incident light 160passes through the semiconductor layers of variable spectral responsephotodetector 100, photons from the incident light 160 are absorbed andgenerate photocurrent in top photodiode 135 and/or bottom photodiode155.

In some configurations, reference to various characteristics ofdepletion region 130, common anode region 140, and/or depletion region150 may refer to the portions of these regions in the photo-activeportion of variable spectral response photodetector 100. For example,the photo-active portion of variable spectral response photodetector100-a may be illustrated in FIG. 1A as the portion bounded by axis linesZ₁ and Z₂ that generally receives incident light 160. Accordingly,reference to characteristics of top photodiode 135, depletion region130, common anode region 140, bottom photodiode 155, and/or depletionregion 150, should not be construed to require that the characteristicholds for the entirety of the device and/or region. For example,depletion region 130 may generally be described as in between depletionregion 150 and the semiconductor surface 122. It should be understoodthat this characteristic may refer to the photo-active portions of theserespective regions.

Absorption depth of incident light in semiconductor materials varies bywavelength. Generally, shorter wavelength light is absorbed at shallowerdepths and longer wavelength light has a deeper absorption depth. Insilicon for example, blue light with a wavelength of 450 nm may have anabsorption depth, defined as a light intensity of 1/e (36%) of itsoriginal value, of approximately 0.3 μm, while green light having awavelength of approximately 530 nm may have an absorption depth ofapproximately 1.0 μm, and red light having a wavelength of approximately700 nm may have an absorption depth of approximately 4.0 p.m. Othersemiconductor materials have similar absorption properties, withabsorption depths varying in the range of sub-micron depths to more thana centimeter for infra-red wavelengths in some semiconductor materials.In the present description, a silicon photodetector is described foroptical responses in the visible range and infra-red regions, however,it should be understood that an appropriate semiconductor may beselected for a particular application based on a desired range ofspectral response and material thickness.

Photons absorbed in the depletion region 130 of the top photodiode 135produce electron-hole pairs as part of the absorption process, i.e. theenergy of the photon separates a negatively charged electron from thesilicon crystal structure making it a free carrier and leaving apositively charged hole in the structure. Described in terms of energy,an electron in the silicon valence band absorbs the photon energy andcrosses the silicon bandgap to the conduction band, leaving behind afree hole. The free electron is separated from the hole by therelatively high electric field in the depletion region 130 and iscollected at the cathode region 132. Similarly, an electron photocurrentis generated in the depletion region 150 of bottom photodiode 155 and iscollected at the backgate cathode region 156. The two hole currents arecombined and collected by the common anode region 140. In someembodiments, the photocurrent at the common anode region may be ignoredsince it contains less information than the separate cathodephotocurrents of the photodiodes 135 and 155, respectively.

In the common anode region 140, approximately half of the electronsgenerated by absorbed photons in the common anode region 140 may diffuseto the depletion region 130, and the remaining electrons may diffuse tothe depletion region 150. In some embodiments, the photodiodes 135 and155 are biased to form a relatively thin common anode region 140, inwhich case common anode region 140 may be modeled as a collectionboundary 145 between depletion region 130 and depletion region 150.Thus, a narrow depletion region 140 may provide more accurate control ofthe spectral response of photodetector 100 because the exact location ofthe boundary between common anode region 140 and depletion regions 130and/or 150 need not be determined Control of the thickness of commonanode region 140 may be used to accurately adjust the depth ofcollection boundary 145, as will be discussed in more detail below.

Because absorption depth varies with wavelength of incident light,spectral response of the top photodiode 135 may be controlled by varyingthe thickness of the depletion region 130, particularly by adjusting thedepth of the collection boundary 145 relative to the semiconductorsurface 122. For example, a relatively small reverse bias voltageapplied to photodiode 135, combined with a relatively larger reversebias voltage applied to photodiode 155 may produce a thin depletionregion 130 that is close to the semiconductor surface and/or cathoderegion 132. With this relatively thin depletion region 130, light havingshorter wavelengths such as blue light may be absorbed in depletionregion 130 while longer wavelength light passes through depletion region130 without being absorbed. The longer wavelength light may be absorbedin the depletion region 150 of the photodiode 155. Increasing thereverse bias voltage applied to photodiode 135 may generally increasethe thickness of the depletion region 130. Similarly, thicknesses of thecommon anode region 140 and/or depletion region 150 may be controlled byvarying the reverse bias voltage applied to photodiode 135 inconjunction with a reverse bias voltage applied to the bottom photodiode155.

In certain embodiments, common anode region 140 may be contacted via twocommon anode contacts. For example, common anode region 140 may becoupled with a first P+ implant region 142 and a second P+ implantregion 144 on opposite sides of the N+ implant cathode region 132. Thefirst and second P+ implant regions 142, 144 may be used to determinethickness of the common anode region 140 based on a resistancemeasurement between the P+ implant regions 142, 144 as described in moredetail below. However, some embodiments have a single P+ implant regionor multiple P+ implant regions that are electrically coupled in thevariable spectral response photodetector 100.

The electrical structure of variable spectral response photodetector 100may include two photodiodes 135 and 155 sharing a common anode regionthat varies in depth, and optionally in thickness, based on biasvoltages applied to the two photodiodes 135 and 155. FIG. 1A illustratesa vertical structure for variable spectral response photodetector 100-awhere depletion region 130 of the top photodiode 135 is formed proximateto the surface 122 of the variable spectral response photodetector 100-aand between cathode region 132 and common anode region 140. In thevertical structure of FIG. 1A, depletion region 150 of photodiode 155 isformed between common anode region 140 and buried N+ cathode region 156.However, other structures are possible for variable spectral responsephotodetector 100 including horizontal photodiode structures. Forexample, depletion regions 130 and 150 and common anode region 140 maybe formed as vertical layers and incident light 160 may be received bythe light detector from a horizontal direction at a vertical planeproximate to depletion region 130.

FIG. 1B shows a top or plan view of variable spectral responsephotodetector 100-a in accordance with various embodiments.Cross-sectional planes may be defined to assist in illustrating crosssections of variable spectral response photodetector 100-a. For example,a cross-sectional plane X₁-X₂ 172 may be described as a left-to-rightcross-section and a cross-sectional plane Y₁-Y₂ 174 may be described asa front-to-back cross section of variable spectral responsephotodetector 100-a. For clarity, light shield layer 180 is notillustrated in FIG. 1B and may be omitted in subsequent figures ofvarious embodiments.

As seen in the plan view, FIG. 1B illustrates that N-Well region 154 maysurround N+ cathode region 132 and P+ common anode regions 142 and 144.N-Well region 154 may include one or more N+ implant regions 152. FIGS.1A and 1B illustrate only one example of possible structures forvariable spectral response photodetector 100-a and other topographiesand/or layouts are within the scope of the present disclosure and willbe readily recognized by one of skill in the art.

As described above, the spectral response of top photodiode 135 andbottom photodiode 155 may depend on respective thicknesses of depletionregion 130 and depletion region 150, which may be defined by a depth ofthe collection boundary 145 between the depletion regions. For example,a relatively thin depletion region 130 may result in a spectral responseof the top photodiode 135 having primarily shorter wavelength componentsand fewer longer wavelength components. In this instance, longerwavelength components not absorbed by the top photodiode 135 may beabsorbed by the bottom photodiode 155, resulting in a primarilylonger-wavelength spectral response of the bottom photodiode 155.Biasing top photodiode 135 and bottom photodiode 155 to have arelatively thicker depletion region 130 may result in more longerwavelengths absorbed by top photodiode 135.

As mentioned above, aspects of various embodiments are directed toaccurately controlling spectral response and/or calibration of anapparatus or system employing a variable spectral response photodetectoraccording to the present disclosure. Referring generally to FIGS. 2A,2B, and 2C, control and/or calibration of spectral response of variablespectral response photodetector 100 using resistance measurement of thecommon anode region is described in more detail, according to variousembodiments. As illustrated in FIGS. 2A, 2B, and 2C, a system employingvariable spectral response photodetector 100 may include controllablevoltage source 235 for applying a bias voltage between N+ cathode region132 and P+ implant regions 142 and 144, controllable voltage source 255for applying a bias voltage between N+ cathode region 152 and P+ implantregions 142 and 144, and resistance measurement unit 245 for measuring aresistance between P+ implant regions 142 and 144. While not illustratedspecifically in FIGS. 2A, 2B, and 2C, the present description willcontinue to refer to the top photodiode 135 and bottom or backgatephotodiode 155 as illustrated schematically in FIG. 1A.

FIG. 2A illustrates a first control state 200-a for a light detectordevice employing a variable spectral response photodetector 100 inaccordance with various embodiments. To establish the first controlstate 200-a, a first voltage may be applied to the top photodiode 135 byvoltage source 235. For example, a relatively small reverse bias voltageor a voltage of zero volts may be applied by voltage source 235.Initially, voltage source 255 may also be set to a relatively smallvoltage or zero volts. Because the top photodiode 135 has a relativelysmall reverse bias, the effective thickness T_(D1-A) of depletion region130 may be relatively thin. Voltage source 255 may then be increaseduntil a desired thickness T_(CA-A) of common anode region 140 isestablished. Thickness of common anode region 140 may be determined bymeasuring resistance between P+ implant regions 142 and 144. Forexample, if the resistivity of the P−epitaxial layer is 10 Ω-cm, theresistance of a 0.1 μm thick common anode region 140 may be 1 MΩ.Resistance between P+ implant regions 142 and 144 may be measured, forexample, by applying a small (e.g., 0.1 volt) voltage differentialbetween contacts of P+ implant regions 142 and 144 and measuring currentflow between the respective contacts.

First control state 200-a may correspond to a first measurement statefor variable spectral response photodetector 100. In embodiments,thickness T_(XA-A) of the common anode region 140 for measurement statesis selected to be relatively thin to reduce the amount of absorbed lightwithin the common anode region 140 and provide improved control of thedepth of collection boundary 145. For example, thickness T_(CA-A) of thecommon anode region 140 for measurement states may be approximately 0.1μm. The actual depth of collection boundary 145 below the semiconductorsurface 122 may be determined by the depth of N+ cathode region 132 andthe thickness T_(D-DA) of depletion region 130. In the first controlstate 200-a, depletion region 150 has a thickness of T_(D2-A) extendingfrom the collection boundary 145 to N+ buried cathode 156. Electronphotocurrent generated in depletion region 130 for variable spectralresponse photodetector 100 in first control state 200-a may be measuredat voltage source 235. Electron photocurrent generated in depletionregion 150 for the first control state 200-a may be measured at voltagesource 255. Photodiode current may be measured at voltage sources 235and 255 using known circuit techniques (e.g., transimpedance amplifiers,etc.).

FIG. 2B illustrates a second control state 200-b for a light detectordevice employing a variable spectral response photodetector 100 inaccordance with various embodiments. Starting from the first controlstate 200-a, the reverse bias voltage applied by voltage source 255 maybe decreased until the thickness of common anode region 140 increases bya desired step size. For example, for a step of 0.1 μm in depth of thecollection boundary 145, the thickness of the common anode region 140may be increased from 0.1 μm (e.g., T_(CA-A)) to 0.2 μm by lowering thevoltage applied by voltage source 255 until the resistance betweencommon anode contacts 142 and 144 decreases from 1.0 MΩ to 0.5 MΩ.

FIG. 2C illustrates a third control state 200-c for a light detectordevice employing a variable spectral response photodetector 100 inaccordance with various embodiments. Third control state 200-c maycorrespond to a second measurement state for variable spectral responsephotodetector 100. Starting from the second control state 200-b, thereverse bias voltage applied by voltage source 235 may be increaseduntil the thickness of the common anode region 140 decreases to again besubstantially equal to thickness T_(CA-A). For example, the reverse biasapplied by voltage source 235 may be increased from control state 200-buntil the resistance between common anode contacts 142 and 144 increasesfrom 0.5 MΩ to 1.0 MΩ. In the third control state 200-c, the thicknessT_(D1-B) of depletion region 130 may be increased by substantially thedesired step size from the thickness T_(D1-A) of depletion region 130 infirst control state 200-a, while the thickness T_(D2-B) of depletionregion 150 may decrease by substantially the desired step size from thethickness T_(D2-A) of depletion region 150 in first control state 200-a.

Starting from the third control state 200-c, the steps described relatedto the second and third control states may be repeated to step the depthof the collection boundary 145 between the depletion region 130 of thetop photodiode 135 and the depletion region 150 of the bottom photodiode150 in controlled increments. Measurements of electron photocurrent ofthe top photodiode 135 and optionally of the bottom photodiode 155 maybe made at each iteration of the third control state (e.g., with thethickness of the common anode region equal to T_(CA-A)). As thethickness of the depletion region 130 of the top photodiode 135 isincreased, the depletion region 130 absorbs more wavelengths of lightand receives additional photocurrent contribution from the absorbedlight. Therefore, this procedure may be used to control the spectralresponse of the photodetector 100 by accurately controlling thethicknesses of the depletion region 130 of the top photodiode 135, thecommon anode region 140, and the depletion region 150 of the bottomphotodiode 155. It is to be noted that increasing the thickness of thedepletion region 130 of the top photodiode 135 while maintaining thethickness of the common anode region equal to a predetermined value,i.e. T_(CA-A), results in a corresponding decrease in thickness of thedepletion region 150 of the bottom photodiode 155. As the depletionregion 150 is reduced in thickness it may absorb fewer wavelengths oflight and thus receive less photocurrent contribution from the absorbedlight. Conversely, decreasing the thickness of the depletion region 130of the top photodiode 135 while maintaining the thickness of the commonanode region equal to a predetermined value, i.e. T_(CA-A), results in acorresponding increase in thickness of the depletion region 150 of thebottom photodiode 150. As the depletion region 150 is increased inthickness it may absorb more wavelengths of light and thus receive morephotocurrent contribution from the absorbed light.

In some embodiments, capacitance of depletion region 130 and/ordepletion region 150 may be used to control the spectral response of thephotodetector 100 instead of or in addition to common anode regionresistance. Capacitance of a diode depletion region generally decreaseswith increasing depletion region thickness. Using this knownrelationship, measured capacitance of the top photodiode 135 and bottomphotodiode 155 may be used to determine thicknesses of depletion region130, common anode region 140, and/or depletion region 150.

In some embodiments, the described procedures for setting the depth ofcollection boundary 145 based on common anode region resistance and/ordepletion region capacitance may be used during an operational mode(e.g., before performing each measurement) of the variable spectralresponse photodetector 100 to sense light intensity across differentranges of wavelengths. In some embodiments, the described procedures areused for calibration of variable spectral response photodetector 100.For example, the procedures described above for setting the depth ofcollection boundary 145 and/or thicknesses of depletion region 130 anddepletion region 150 may be used, and the calibrated drive voltages forthe top and bottom photodiodes at various thicknesses of depletionregions 130 and 150 and/or depths of the collection boundary 145 may bestored in memory or non-volatile storage. The stored values may then beused in operation of the variable spectral response photodetector 100.Variable spectral response photodetectors 100 may be individuallycalibrated using the described procedures to reduce or eliminateresponse variations due to manufacturing process tolerances.Alternatively, calibration may be performed periodically or before eachset of measurements based on the procedures for setting the depth of thecollection boundary 145 and/or thicknesses of depletion regions 130and/or 150 as described above.

FIG. 3 illustrates example voltage settings for the top and bottomphotodiodes with a consistent (e.g., 0.1 μm) common anode regionthickness for various depths of the collection boundary 145, where thex-axis represents the depth of the collection boundary 145 and they-axis represent the respective photodiode reverse bias in volts. Curves335 and 355 illustrate the respective reverse bias voltages of the topphotodiode 135 and bottom photodiode 155 that may be used to set aparticular collection boundary depth, at a predetermined thickness ofthe common anode region 140, for an example variable spectral responsephotodetector 100. FIG. 3 illustrates that a voltage range of about 20Vmay be sufficient to provide a sufficient range of collection boundarydepths for many visible light applications using a silicon substrate.

From the photocurrent measurements at various depths of the collectionboundary 145, the amount of light absorbed from various wavelengths canbe determined. The generated photocurrent of the top photodiode at adepth of the collection boundary, d, may be given by:

$I_{td} = {\sum\limits_{k = 0}^{n}{A_{k}\left( {1 - ^{({{- d}/\alpha_{k}})}} \right)}}$

Where A_(k) is the unknown incident light power at a wavelength k; α_(k)is the absorption depth of wavelength λ_(k) for the constituentmaterial; and d is defined as the distance from semiconductor surface122 to the collection boundary 145. The generated photocurrent of thebottom or backgate photodiode 155 at a collection boundary depth, d, maybe given by:

$I_{bd} = {\sum\limits_{k = 0}^{n}{A_{k} \cdot ^{({{- d}/\alpha_{k}})}}}$

To determine the incident light power at a specific wavelength or over arange of wavelengths, the electron photocurrent response of the topphotodiode 135 and/or bottom photodiode 155 may be measured at multiplecollection boundary depths and the values of A_(k) may be calculated bymatrix calculation. The calculated values of A_(k) may includemultiplying factors for surface reflectance and quantum efficiency.These factors may be accounted for by known techniques to determine theincident light amplitude for the various spectral components. Spectralresponse data including incident light power at N wavelengths (orwavelength ranges) may be generated by measuring electron photocurrentresponse at a number M thicknesses of the depletion region of the topphotodiode, where M≧N.

Because shorter wavelength light may be absorbed in regions relativelyclose to the surface of the variable spectral response photodetector100, resolution of shorter wavelength light may be limited by thethickness of the N+ cathode region 132. In some embodiments, thethickness of N+ cathode region 132 may be reduced to improve spectralresolution at shorter wavelengths. In some embodiments, resolution ofthe variable spectral response photodetector at shorter wavelengths maybe improved through the use of various alternative cathode structuresfor the top photodiode 135.

FIGS. 4A and 4B illustrate a variable spectral response photodetector100-b with an alternative cathode structure for the top photodiode 135in accordance with various embodiments. FIG. 4B illustrates a plan viewof variable spectral response photodetector 100-b with left-to-rightcross-sectional plane X₁-X₂ 172 and front to back cross-sectional planeY1-Y2 174 illustrated for reference. FIG. 4A may illustrate a crosssection of variable spectral response photodetector 100-b incross-sectional plane X₁-X₂ 172.

Similarly to photodetector 100-a, variable spectral responsephotodetector 100-b may have a two-layer structure with a P−substrate110 and P−epitaxial layer 120. N+ buried cathode 456 may be formed inP−substrate 110 and connected to N+ implant region 152 through N-Well154 to form, generally, the cathode region of the bottom photodiode 155.The P−epitaxial layer 120 may have an inversion region close to thesurface 122, which may form a thin (e.g., approximately 0.01 μm) N+cathode region between N+ cathode implant regions 432 for top photodiode135. P+ anode regions 442 and 444 may provide anode connections to thetop photodiode 135 and bottom photodiode 155. FIG. 4A illustrates thatthe top photodiode 135 and bottom photodiode 155 can be biased such thatthe common anode region 140 is closer to the semiconductor surface 422than may be possible with the N+ cathode implant region of the variablespectral response photodetector 100-a illustrated in FIGS. 1A and 1B.

FIGS. 5A, 5B and 5C illustrate a variable spectral responsephotodetector 100-c employing a metal oxide semiconductor (MOS) cathodestructure in accordance with various embodiments. FIG. 5C may illustratea plan view of variable spectral response photodetector 100-c withleft-to-right cross-sectional plane X₁-X₂ 172 and front to backcross-sectional plane Y₁-Y₂ 174 illustrated for reference. FIG. 5A mayillustrate a cross section of variable spectral response photodetector100-c in plane cross-sectional plane X₁-X₂ 172. FIG. 5B may illustrate across section of variable spectral response photodetector 100-c in planecross-sectional plane Y₁-Y₂ 174.

Similarly to photodetectors 100-a and 100-b, variable spectral responsephotodetector 100-c may have a two-layer structure with a P−substrate110 and P−epitaxial layer 120. N+ buried cathode 156 may be formed inP−substrate 110 and connected to N+ implant region 152 through N-Well154 to form, generally, the cathode region of the bottom photodiode 155.Instead of N+ implant region 132, transparent gate electrode 534 and N+source/drain region(s) 536 may form a MOS structure with the MOS channelunder transparent gate electrode 534 forming the effective cathoderegion 532 of the top photodiode 135. The MOS structure formed bytransparent gate electrode 534 and N+ source/drain region(s) 536 mayhave a thinner channel than is possible with an N+ implant layer,thereby potentially allowing depletion region 130 of the top photodiode135 to form closer to the semiconductor surface 522 than may be possibleusing the photodetector structure illustrated in FIG. 1A.

To form the cathode region for top photodiode 135, the transparent gateelectrode 534 may be biased to produce a very thin (e.g., substantiallyzero depletion depth) cathode region 532 under the transparent gateelectrode. For example, the transparent gate electrode 534 may be biasedin the sub-threshold region to produce the flat band or zero depletiondepth condition. As the bias voltage of the transparent gate electrode534 relative to the anode (e.g., anode implant regions 542, 544) becomesmore positive from the flat band condition, the depletion layer getsdeeper until an inversion layer of electrons forms at the surfacecreating a very thin (e.g., <0.1 μm) conducting layer (illustrated inthe short dashed lines of FIG. 5B) substantially similar to theconducting layer of a MOS field effect transistor (FET). For example,the inversion layer may form at the gate threshold voltage of theequivalent MOSFET structure. This thin conducting layer may form theeffective cathode region 532 of variable spectral response photodetector100-c.

If the transparent gate electrode 534 is forward biased further relativeto the anode, the depletion layer depth stops growing and the increasedcharge across the gate capacitor derived from increasing the gatevoltage may go entirely into the inversion layer. However, because thethin conducting layer (substantially equivalent to an N+ doping layer)is connected to the N+ source/drain regions 536, the voltage of the N+source/drain regions 536 and the transparent gate electrode 534 can beincreased simultaneously to continue increasing the thickness of thedepletion region 130 of the top photodiode 135. By substantiallymaintaining the voltage difference between the transparent gateelectrode 534 and the N+ source/drain regions 536 at approximately thethreshold voltage of the MOS cathode structure, the thickness of thedepletion region 130 can be increased without substantially increasingthe thickness of the cathode depletion region 532 formed by the MOSchannel. Therefore, the thickness of the depletion region 130 can beaccurately controlled to vary the spectral response of the topphotodiode 135. The thickness of the depletion region 150 of the bottomphotodiode 155 and the thickness of the common anode region 140 may becontrolled using similar techniques to those described above.

FIG. 6A illustrates example electron photocurrent response curves of thetop photodiode of variable spectral response photodetector 100 atvarious collection boundary layer depths, according to variousembodiments. In FIG. 6A, the x-axis represents wavelength in nanometersand the y-axis represents an example of photocurrent power response ofthe top photodiode 135 in amperes per watt of received light.

FIG. 6B illustrates example photocurrent response curves of the backgateor bottom photodiode 155 at various collection boundary depths,according to various embodiments. In FIG. 6B, the x-axis representswavelength in nanometers and the y-axis represents an example ofphotocurrent power response for the bottom photodiode 155 in amperes perwatt of received light.

FIG. 7 illustrates an example of using variable spectral responsephotodetector 100 to sense light intensity at different wavelengthregions. Light intensity for a range of wavelengths (e.g., red, green,blue, UV, infrared, etc.) may be determined by subtracting the measuredphotocurrent of the top photodiode with the depletion region 130 set ata first thickness from the measured photocurrent of the top photodiodewith the depletion region 130 set at a second thickness. For example,according to the photocurrent response of the top photodiode at variousthicknesses of depletion region 130 shown in FIG. 7, the blue response710 may be obtained for incident light by subtracting the measuredphotocurrent with the depletion region 130 set at a thickness ofapproximately 0.2 μm from the measured photocurrent with the depletionregion 130 set at a thickness of approximately 0.8 μm. The greenresponse 720 may be obtained by subtracting the measured photocurrentwith the depletion region 130 set at a thickness of approximately 0.8 μmfrom the measured photocurrent with the depletion region 130 set at athickness of approximately 3.5 μm. The red response 730 may be obtainedby subtracting the measured photocurrent with the depletion region 130set at a thickness of approximately 3.0 μm from the measuredphotocurrent with the depletion region 130 set at a thickness ofapproximately 5.0 μm.

As can be seen in FIG. 7, response curves obtained by subtracting afirst measured photocurrent of the top photodiode at a first thicknessfrom a second measured photocurrent of the top diode at a secondthickness may have a substantial response tail in the red and infraredregions. To compensate for the absorbed red and/or infrared light seenin the tail of the response curves, the response curves for the variouswavelength ranges may be combined according to a second set ofcalculations based on measured photocurrent response of the topphotodiode or the backgate photodiode.

For example, an adjusted blue response curve 715 may be obtained bysubtracting a portion of the unadjusted green response. In the exampleshown in FIG. 7, 28% of the unadjusted green photocurrent is subtractedto obtain the adjusted blue response 715. An adjusted green response 725may be obtained by subtracting a portion of the unadjusted redphotocurrent. In the example shown in FIG. 7, 150% of the unadjusted redphotocurrent is subtracted to obtain the adjusted green response 725. Anadjusted red response 735 may be obtained by subtracting a portion ofthe backgate photocurrent. In the example shown in FIG. 7, 65% of thebackgate photocurrent, measured at a thickness of the depletion region130 of 4.5 μm, is subtracted to obtain the adjusted red response 735.These adjustments are merely examples and other calculations usingmeasured photocurrent of the top photodiode and/or backgate photodiodemay be used to obtain other response curves of desired response for agiven wavelength range. The portions utilized may be determined by acalibration stage in cooperation with calibrated testing equipment, asis known to those skilled in the art, with the factors stored in memory,or non-volatile storage.

Another example of using variable spectral response photodetector 100 tosense light intensity at different wavelength regions may find use inadapting a light sensor to the human eye response. For example, thevisual sensitivity of the human eye has different luminosity functionsbased on daytime-adapted (photopic) and darkness-adapted (scotopic)conditions. Variable spectral response photodetector 100 may be used toproduce light measurements that dynamically account for photopic andscotopic response of the human eye. For example, the techniquesdescribed above with reference to FIG. 7 may be used to dynamicallyadjust the spectral response of variable spectral response photodetector100 based on the total incident light level.

Turning to FIG. 8A, a cross-sectional view 800-a of a portion of avariable spectral response photodetector 100 is illustrated inaccordance with various embodiments. Cross-sectional view 800-a may be,for example, a view of cross-sectional plane Y₁-Y₂ 174 of variablespectral response photodetector 100-a illustrated in FIGS. 1A and 1B.When the reverse bias voltage of the top photodiode 135 is relativelysmall, charge 824 in an oxide layer 124 near the surface of thephotodiode structure may create a parasitic FET that may cause buildupof surface charge 826 which may allow leakage current (I_(L)) betweenthe cathode regions of the top photodiode 135 and the bottom photodiode155 and/or between the cathode and anode regions of the top photodiode135 and/or backgate photodiode 155. This leakage current may reduce theaccuracy of photocurrent measurements of the top photodiode 135 and/orbottom photodiode 155.

FIG. 8B illustrates a cross-sectional view 800-b of a portion of avariable spectral response photodetector 100-d employing an implantregion to reduce surface cathode leakage in accordance with variousembodiments. Variable spectral response photodetector 100-d may includean implant region 838 in between the cathode region of the topphotodiode 135 (e.g., N+ implant region 132) and the cathode region ofthe bottom photodiode 155 (e.g., N-well 154). Implant region 838 may beused to increase the surface doping to prevent the natural inversionlayer illustrated in FIG. 8A that provides a leakage channel in betweenthe cathode regions of the top photodiode 135 and the bottom photodiode155. For example, implant region 838 may be a lightly doped P-wellregion. Implant region 838 may be biased, or, in embodiments, may beunconnected to any bias voltage.

FIG. 8C illustrates a cross-sectional view 800-c of a portion of avariable spectral response photodetector 100-e that employs an edge gateto limit surface leakage in accordance with various embodiments.Variable spectral response photodetector 100-e may include an edge gate834 in between the cathode region of the top photodiode 135 (e.g., N+implant region 132) and the cathode region of the bottom photodiode 155(e.g., N-well 154). Edge gate 834 may be biased to prevent the formationof an inversion layer under edge gate 834, cutting off the leakage pathbetween the photodiode cathode regions. For example, edge gate 834 maybe biased to zero volts, or, in embodiments, negatively biased withrespect to the P−epi region 820 by negatively biasing edge gate 834 withrespect to the shared cathode of the top photodiode 135 and the bottomphotodiode 155 contacted through P+ implant regions 142 and/or 144.

FIG. 9 illustrates a light detector apparatus 900, according to variousembodiments. Light detector apparatus 900 may include a variablespectral response photodetector 910 that receives incident light 960 anda detector driver module 920 that is coupled to variable spectralresponse photodetector 910 and controls the spectral response ofvariable spectral response photodetector 910. Variable spectral responsephotodetector 910 may be an example of aspects of variable spectralresponse photodetectors 100 as described above with reference to FIG.1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 2C, FIG. 4A, FIG. 4B, FIG. 5A, FIG.5B, FIG. 5C, FIG. 8A, FIG. 8B, and/or FIG. 8C. For example, variablespectral response photodetector 910 may include a first photodiode and asecond photodiode with a variable depth common anode region.

In embodiments, the first photodiode includes a first depletion regionbetween the light acceptance surface and a common anode region thatabsorbs a first portion of the incident light. The first photodiode maygenerate a photocurrent responsive to the absorbed first portion ofincident light. The variable spectral response photodetector 910 mayinclude a second photodiode with a second depletion region opposite ofthe light acceptance surface from the common anode region. The seconddepletion region may absorb a second portion of the incident light in asecond depletion region. The second photodiode may generate aphotocurrent responsive to the absorbed second portion of incidentlight.

Detector driver module 920 may apply bias voltages to the first andsecond photodiodes of variable spectral response photodetectors 910 tovary the spectral response of photocurrent generated by the photodiodes.Detector driver module 920 may measure photocurrent of the first and/orsecond photodiodes to determine spectral components of incident light960. For example, detector driver module 920 may apply a first biasvoltage to the first photodiode to set the thickness of the firstdepletion region to a first predetermined thickness. Detector driver 920may apply a second bias voltage to the first photodiode to control thethickness of the first depletion region to a second predeterminedthickness. Adjusting the thickness of the first depletion region mayvary the spectral response of variable spectral response photodetector910. As described above, adjusting the thickness of the first depletionregion, while maintaining a fixed common anode region thickness, mayresult in an inverse adjustment of the second depletion region.

Detector driver module 920 may make multiple measurements ofphotocurrent from the first and/or second photodiodes to determinespectral components of the incident light 960. For example, detectordriver module 920 may apply multiple voltage bias-points to the firstphotodiode corresponding to various thicknesses of the first depletionregion. The thickness of the common anode region may be controlled bythe detector driver module 920 to be substantially the same thickness ateach bias-point. The detector driver module 920 may determine spectralcomponents of the incident light for multiple wavelengths and/orwavelength ranges by matrix calculations based on the measuredphotocurrents and light absorption information as described above.

FIG. 10 illustrates a detector driver module 1020 for use in a lightdetector apparatus or system, according to various embodiments. Detectordriver module 1020 may be an example of one or more aspects of detectordriver module 920 described with reference to FIG. 9. Detector drivermodule 1020 may include a first photodiode driver block 1035, a secondphotodiode driver block 1055, and/or a processor block 1040. Detectordriver module 1020 may include a resistance measurement unit 1045 formeasuring resistance of the common anode region of a variable spectralresponse photodetector. Photodiode driver blocks 1035 and/or 1055 may beoperative to apply voltage to photodiodes of a photodetector and measurephotocurrent from the photodetector. Photodiode driver blocks 1035and/or 1055 may include various circuit elements for driving biasvoltages to photodiodes and measuring photocurrent from the photodiodes,as is known in the art (e.g., reference voltage generators, amplifiers,transimpedance amplifiers, integrating transimpedance amplifiers, etc.).

Components of detector driver modules 920 and/or 1020, may, individuallyor collectively, be implemented with one or more Application SpecificIntegrated Circuits (ASICs) adapted to perform some or all of theapplicable functions in hardware. Alternatively, the functions may beperformed by one or more other processing units (or cores), on one ormore integrated circuits. In other embodiments, other types ofintegrated circuits may be used (e.g., Structured/Platform ASICs, FieldProgrammable Gate Arrays (FPGAs), and/or other Semi-Custom ICs), whichmay be programmed in any manner known in the art. The functions of eachunit may also be implemented, in whole or in part, with instructionsembodied in a memory, formatted to be executed by one or more general orapplication-specific processors.

FIG. 11 illustrates a light detector apparatus 1100 employing a variablespectral response photodetector, according to various embodiments. Lightdetector apparatus 1100 may include a variable spectral responsephotodetector 1110, a first photodiode driver block 1135, a secondphotodiode driver block 1155, and/or a processor block 1140. Lightdetector apparatus 1100 may also include a resistance measurement block1145 for measuring resistance of a common anode region of the variablespectral response light detector 1110. Variable spectral responsephotodetector 1110 may be an example of aspects of variable spectralresponse photodetectors 100 as described above with reference to FIG.1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 2C, FIG. 4A, FIG. 4B, FIG. 5A, FIG.5B, FIG. 5C, FIG. 8A, FIG. 8B, and/or FIG. 8C. For example, variablespectral response photodetector 1110 may include a first photodiode anda second photodiode with a variable depth collection boundary.

Light detector apparatus 1100 illustrates that various componentsrelated to control of variable spectral response photodetector 1110 maybe integrated within a single component, integrated circuit substrate,and/or package. In embodiments, variable spectral response lightdetector 1110, photodiode driver block 1135, and photodiode driver block1155 are integrated into a single photodetector integrated circuit 1150.The photodetector integrated circuit 1150 may receive digital or analogcontrol signals and output digital or analog representations of measuredphotocurrent from voltage variable spectral response photodetector 1110.In embodiments, the photodetector integrated circuit 1150 includesresistance measurement block 1145 that measures resistance betweenelectrical contacts of the common anode region of voltage variablespectral response light detector 1110 for operation and/or calibrationof the photodetector.

Turning to FIG. 12, a processor block 1200 is illustrated in accordancewith various embodiments. Processor block 1200 may include processor1205, storage device 1210, and/or memory 1215. The processor 1205 mayinclude an intelligent hardware device, e.g., a central processing unit(CPU) such as those made by Intel® Corporation or AMD®, amicrocontroller, an application-specific integrated circuit (ASIC), etc.The memory 1215 may include random access memory (RAM) and/or read-onlymemory (ROM).

The storage device 1210 may include ROM, a solid-state storage device(SSD), a magnetic storage device (hard drive, etc.) and/or the like. Thestorage device 1210 may also store computer-readable,computer-executable software code 1212 containing instructions that areconfigured to, when executed, cause the processor 1205 to performvarious functions described herein (e.g., setting photodiode biasvoltages, measuring photocurrent, etc.). Alternatively, the softwarecode 1212 may not be directly executable by the processor module 1205but be configured to cause the computer, e.g., when compiled andexecuted, to perform functions described herein. The storage device 1210may additionally store information 1214 associated with calibrationand/or drive settings for a variable spectral response photodetector.The information 1214 may be stored in an appropriate data structure(e.g., a look-up-table (LUT) and the like) for use by the processor 1205in setting drive voltages, determining spectral components from measuredphotocurrent information, and other control functions.

FIG. 13 illustrates a color analyzer system 1300 employing a variablespectral response photodetector, according to various embodiments. Coloranalyzer system 1300 may include variable spectral responsephotodetector 1310, a detector driver module 1320, a computer orapplication specific module 1330, and a display screen or user interface1340. Color analyzer system 1300 may receive incident light 1360 from alight source or object 1350 and be operable to determine one or morespectral components of the incident light 1360. Color analyzer system1300 may provide graphical or textual output of the spectral componentson user interface 1340. Variable spectral response photodetector 1310may include aspects of variable spectral response light detectors 100 asdescribed above with reference to FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B,FIG. 2C, FIG. 4A, FIG. 4B, FIG. 5A, FIG. 5B, FIG. 5C, FIG. 8A, FIG. 8B,and/or FIG. 8C. For example, variable spectral response photodetector1310 may include a first photodiode and a second photodiode with avariable depth common anode region. Detector driver module 1320 may bean example of one or more aspects of detector driver module 920 and/or1020 described with reference to FIG. 9 and/or FIG. 10.

Computer or application specific module 1330 may be a general purposecomputer or application specific computer module that is operable tocontrol higher level functions related to color analyzer system 1300such as display of information on user interface 1340, calibration ofdetector driver module 1320 and/or light detector module 1310, storingof spectral component information, etc.

FIG. 14 illustrates a spectrometer system 1400 employing a variablespectral response photodetector, according to various embodiments.Spectrometer system 1400 may include variable spectral responsephotodetector 1410, a detector driver module 1420, a computer orapplication specific module 1430, and a display screen or user interface1440. Variable spectral response photodetector 1410 may include aspectsof variable spectral response photodetectors 100 as described above withreference to FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 2C, FIG. 4A, FIG.4B, FIG. 5A, FIG. 5B, FIG. 5C, FIG. 8A, FIG. 8B, and/or FIG. 8C. Forexample, variable spectral response photodetector 1410 may include afirst photodiode and a second photodiode with a variable depth commonanode region. Detector driver module 1420 may be an example of one ormore aspects of detector driver module 920 and/or 1020 described withreference to FIG. 9 and/or FIG. 10.

Spectrometer system 1400 may receive incident light 1460 from a lightsource or object 1450 and be operable to determine spectral componentsof the incident light 1460 across a range of wavelengths. Spectrometersystem 1400 may provide graphical or textual output of the spectralcomponents on user interface 1440. For example, spectrometer system 1400may scan a wavelength range and graphically illustrate incident lightintensity according to wavelength. In embodiments, detector drivermodule 1420 is operable to apply bias voltages to a first and secondphotodiode of variable spectral response photodetector 1410 to controlspectral response of the variable spectral response photodetector 1410.Detector driver module 1420 may apply predetermined bias voltages to thefirst and/or second photodiodes and measure photocurrent of the firstand/or second photodiodes at multiple voltage bias points. Detectordriver module 1420 and/or application module 1430 may determine spectralcomponents of the incident light 1460 at multiple wavelengths orwavelength regions from the measured photocurrent at the multiplevoltage bias points. Spectrometer 1400 may be used in spectroscopyapplications by passing incident light 1460 through a sample to beanalyzed.

Turning to FIG. 15, a flow diagram of a method 1500 for sensing incidentlight received at a light acceptance surface is illustrated inaccordance with various embodiments. Method 1500 may be implementedutilizing aspects of light detectors and/or variable spectral responsephotodetectors including, but not limited to, those illustrated in FIG.1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 2C, FIG. 4A, FIG. 4B, FIG. 5A, FIG.5B, FIG. 5C, FIG. 8A, FIG. 8B, and/or FIG. 8C.

At block 1505 of method 1500, a first bias voltage may be applied to afirst photodiode that absorbs a first portion of incident light in afirst depletion region between the light acceptance surface and a seconddepletion region of a second photodiode, the first photodiode generatinga photocurrent responsive to the absorbed first portion of incidentlight, a thickness of the first depletion region controlled at least inpart based on the first bias voltage. The first bias voltage may beselected based on a desired spectral response for sensing incidentlight. The first bias voltage may be selected based on resistancemeasurements of the common anode region, capacitance measurements of thefirst depletion region, and/or calibrated voltage settings determinedaccording to calibration procedures described with reference to FIGS.2A, 2B, and/or 2C.

At block 1510, the photocurrent of the first photodiode at the firstbias voltage may be measured to obtain a first photocurrent measurement.At block 1515, a second bias voltage may be applied to the firstphotodiode, the thickness of the first depletion region controlled atleast in part based on the second bias voltage. The second bias voltagemay be selected based on the desired spectral response. The second biasvoltage may be selected based on resistance measurements of the commonanode region, capacitance measurements of the first depletion region,and/or calibrated voltage settings determined according to calibrationprocedures described with reference to FIGS. 2A, 2B, and/or 2C.

At block 1520, photocurrent of the first photodiode at the second biasvoltage may be measured to obtain a second photocurrent measurement. Atblock 1525, a spectral component of the incident light may be determinedbased at least in part on the first photocurrent measurement and thesecond photocurrent measurement.

Turning to FIG. 16, a flow diagram of a method 1600 for sensing incidentlight received at a light acceptance surface is illustrated inaccordance with various embodiments. Method 1600 may be implementedutilizing aspects of light detectors and/or variable spectral responsephotodetectors 100 including, but not limited to, those illustrated inFIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 2C, FIG. 4A, FIG. 4B, FIG. 5A,FIG. 5B, FIG. 5C, FIG. 8A, FIG. 8B, and/or FIG. 8C.

At block 1605 of method 1600, a first bias voltage may be applied to afirst photodiode that absorbs a first portion of the incident light in afirst depletion region between the light acceptance surface and a commonanode region, the first photodiode generating a photocurrent responsiveto the absorbed first portion of incident light, the first bias voltagecontrolling a thickness of the first depletion region to substantiallyequal a first predetermined thickness. For example, the firstpredetermined thickness may be selected based on a desired spectralresponse for sensing incident light. The first bias voltage may beselected based on resistance measurements of the common anode region,capacitance measurements of the first depletion region, and/orcalibrated voltage settings determined according to calibrationprocedures described with reference to FIGS. 2A, 2B, and/or 2C.

At block 1610, a second bias voltage may be applied to a secondphotodiode that absorbs a second portion of the incident light in asecond depletion region opposite of the light acceptance surface fromthe common anode region, the second bias voltage controlling a thicknessof the common anode region to substantially equal a second predeterminedthickness. For example, the second predetermined thickness maycorrespond to a predetermined thickness of the common anode region usedfor calibration. The second bias voltage may be selected based onresistance measurements of the common anode region, capacitancemeasurements of the second depletion region, and/or calibrated voltagesettings determined according to calibration procedures described withreference to FIGS. 2A, 2B, and/or 2C.

Blocks 1615, 1620, and 1625 of method 1600 illustrate a process formaking multiple photocurrent measurements at multiple bias points of thefirst bias voltage and the second bias voltage. At block 1615, the firstbias voltage is modified to step the thickness of the first depletionregion. Modification of the first bias voltage may be performed based onresistance measurements of the common anode region, capacitancemeasurements of the first depletion region, and/or calibrated voltagesettings determined according to calibration procedures described withreference to FIGS. 2A, 2B, and/or 2C. At block 1620, the second biasvoltage may be modified to control the thickness of the common anoderegion to substantially equal the predetermined thickness of stage 1610.At block 1625, photocurrent of the first photodiode is measured toobtain a first photocurrent measurement, and photocurrent of the secondphotodiode may be measured to obtain a second photocurrent measurement.

At block 1630, spectral components of the incident light are determinedbased on the photocurrent measurements from repeated iterations ofblocks 1615, 1620, and 1625. For example, spectral components atmultiple wavelengths or ranges of wavelengths may be determined based onthe multiple photocurrent measurements obtained at block 1625. Forexample, matrix calculation of spectral components may be performedbased on the multiple photocurrent measurements and absorption depthinformation as described in more detail above.

It should be noted that the methods, systems and devices discussed aboveare intended merely to be examples. It must be stressed that variousembodiments may omit, substitute, or add various procedures orcomponents as appropriate. For instance, it should be appreciated that,in alternative embodiments, the methods may be performed in an orderdifferent from that described, and that various steps may be added,omitted or combined. Also, features described with respect to certainembodiments may be combined in various other embodiments. Differentaspects and elements of the embodiments may be combined in a similarmanner. Also, it should be emphasized that technology evolves and, thus,many of the elements are exemplary in nature and should not beinterpreted to limit the scope of the invention.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, it will be understood by oneof ordinary skill in the art that the embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, algorithms, structures, and techniques have been shownwithout unnecessary detail in order to avoid obscuring the embodiments.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flow diagram or block diagram. Although each maydescribe the operations as a sequential process, many of the operationscan be performed in parallel or concurrently. In addition, the order ofthe operations may be rearranged. A process may have additional stepsnot included in the figure.

Moreover, as disclosed herein, the term “memory” or “memory unit” mayrepresent one or more devices for storing data, including read-onlymemory (ROM), random access memory (RAM), magnetic RAM, core memory,magnetic disk storage mediums, optical storage mediums, flash memorydevices or other computer-readable mediums for storing information. Theterm “computer-readable medium” includes, but is not limited to,portable or fixed storage devices, optical storage devices, wirelesschannels, a sim card, other smart cards, and various other mediumscapable of storing, containing or carrying instructions or data.

Furthermore, embodiments may be implemented by hardware, software,firmware, middleware, microcode, hardware description languages, or anycombination thereof. When implemented in software, firmware, middlewareor microcode, the program code or code segments to perform the necessarytasks may be stored in a computer-readable medium such as a storagemedium. Processors may perform the necessary tasks.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. For example, the above elements may merely be a component ofa larger system, wherein other rules may take precedence over orotherwise modify the application of the invention. Also, a number ofsteps may be undertaken before, during, or after the above elements areconsidered. Accordingly, the above description should not be taken aslimiting the scope of the invention.

What is claimed is:
 1. An apparatus for sensing incident light, theapparatus comprising: a light detector that receives the incident lightat a light acceptance surface, comprising: a first photodiode thatabsorbs a first portion of the incident light in a first depletionregion, the first photodiode generating a photocurrent responsive to theabsorbed first portion of incident light; and a second photodiode thatabsorbs a second portion of the incident light in a second depletionregion, the first depletion region between the second depletion regionand the light acceptance surface; and a detector driver module coupledwith the light detector, the detector driver module configured to: applya first bias voltage to the first photodiode, a thickness of the firstdepletion region controlled at least in part based on the first biasvoltage; apply a second bias voltage to the first photodiode differentthan the first bias voltage, the thickness of the first depletion regioncontrolled at least in part based on the second bias voltage; measurethe photocurrent of the first photodiode at each of the first and secondbias voltages to obtain at least two photocurrent measurements; anddetermine a spectral component of the incident light based at least inpart on the at least two photocurrent measurements.
 2. The apparatus ofclaim 1, the detector driver module further configured to: apply a thirdbias voltage to the second photodiode while applying the first biasvoltage to the first photodiode, the thickness of the second depletionregion controlled at least in part based on the third bias voltage;measure the photocurrent of the second photodiode at the third biasvoltage; apply a fourth bias voltage to the second photodiode whileapplying the second bias voltage to the first photodiode, the fourthbias voltage different from the third bias voltage, the thickness of thesecond depletion region controlled at least in part based on the fourthbias voltage; and determine the spectral component of the incident lightfurther based at least in part on the photocurrent measurements at thethird and fourth bias voltages.
 3. The apparatus of claim 2, wherein thethird and fourth bias voltages are selected such that a thickness of acommon anode region between the first depletion region and the seconddepletion region is substantially the same when the first and secondbias voltages are applied.
 4. The apparatus of claim 3, wherein thethickness of the common anode region is controlled at least in partresponsive to a resistance of the common anode region.
 5. The apparatusof claim 1, wherein the detector driver module is configured to controlthe thickness of the first depletion region at each of the first andsecond bias voltages based at least in part on a capacitance of thefirst depletion region.
 6. The apparatus of claim 1, wherein thedetector driver module is configured to: measure a photocurrent of thesecond photodiode responsive to the absorbed second portion of theincident light at one or more of the first and second bias voltages toobtain one or more backgate photocurrent measurements; and determine asecond spectral component of the incident light based at least in parton the at least two photocurrent measurements and the one or morebackgate photocurrent measurements.
 7. The apparatus of claim 1, whereinthe detector driver module comprises: a first voltage control modulecoupled with the first photodiode; a first current measurement modulecoupled with the first photodiode; and a processor module coupled withthe first voltage control module and the first current measurementmodule, the processor module configured to determine the spectralcomponent of the incident light based at least in part on the at leasttwo photocurrent measurements via the first current measurement moduleand light absorption depth information.
 8. The apparatus of claim 6,wherein the detector driver module further comprises: a second voltagecontrol module coupled with the second photodiode; and a second currentmeasurement module coupled with the second photodiode, wherein theprocessor module is further coupled with the second voltage controlmodule and the second current measurement module, the processor modulefurther configured to determine the spectral component of the incidentlight based at least in part on photocurrent measurements via the secondcurrent measurement module at each of the first and second biasvoltages.
 9. A method for sensing incident light received at a lightacceptance surface, the method comprising: applying a first bias voltageto a first photodiode that absorbs a first portion of the incident lightin a first depletion region between the light acceptance surface and asecond depletion region of a second photodiode, the first photodiodegenerating a photocurrent responsive to the absorbed first portion ofincident light, a thickness of the first depletion region controlled atleast in part based on the first bias voltage; measuring thephotocurrent of the first photodiode at the first bias voltage to obtaina first photocurrent measurement; applying a second bias voltagedifferent than the first bias voltage to the first photodiode, thethickness of the first depletion region controlled at least in partbased on the second bias voltage; measuring the photocurrent of thefirst photodiode at the second bias voltage to obtain a secondphotocurrent measurement; determining a spectral component of theincident light based at least in part on the first photocurrentmeasurement and the second photocurrent measurement.
 10. The method ofclaim 9, further comprising: applying a third bias voltage to the secondphotodiode while applying the first bias voltage to the firstphotodiode, a thickness of the second depletion region controlled atleast in part based on the third bias voltage; measuring thephotocurrent of the second photodiode at the third bias voltage toobtain a third photocurrent measurement; applying a fourth bias voltageto the second photodiode while applying the second bias voltage to thefirst photodiode, the fourth bias voltage different from the third biasvoltage, the thickness of the second depletion region controlled atleast in part based on the fourth bias voltage; and determining thespectral component of the incident light further based at least in parton the third photocurrent measurement and the fourth photocurrentmeasurement.
 11. The method of claim 9, wherein the applying the thirdand fourth bias voltages comprises: controlling a thickness of a commonanode region between the first depletion region and the second depletionregion to be substantially equal when the first and second bias voltagesare applied.
 12. The method of claim 11, wherein the thickness of thecommon anode region is controlled at least in part responsive to aresistance of the common anode region.
 13. The method of claim 9,wherein the applying the first and second bias voltages comprises:controlling the thickness of the first depletion region at each of thefirst and second bias voltages based at least in part on a capacitanceof the first depletion region.
 14. The method of claim 10, wherein theapplying the third and fourth bias voltages comprises: controlling thethickness of the second depletion region at each of the third and fourthbias voltages based at least in part on a capacitance of the seconddepletion region.
 15. The method of claim 9, further comprising:measuring photocurrent at a first plurality of bias points byiteratively: stepping the thickness of the first depletion region by apredetermined step thickness by, at least in part, modifying the secondbias voltage; and measuring the photocurrent of the first photodiode;and determining an amount of received light at a second plurality ofwavelength ranges by solving a matrix calculation based at least in parton the measured photocurrent at the first plurality of bias points andlight absorption depth information.
 16. The method of claim 15, whereina number of photocurrent measurements measured at the first plurality ofbias points is greater than a number of wavelength ranges of the secondplurality of wavelength regions.
 17. The method of claim 15, furthercomprising: adjusting the matrix calculation to adjust a calculatedspectral response based at least in part on a metric of the measuredphotocurrent at one or more bias points.
 18. The method of claim 17,wherein the metric of the measured photocurrent comprises a combinedincident light level.
 19. A photodetector, comprising: a firstphotodiode that absorbs a first portion of incident light in a firstdepletion region, the first photodiode generating a photocurrentresponsive to the absorbed first portion of incident light; and a secondphotodiode that absorbs a second portion of the incident light in asecond depletion region, the absorbed second portion comprises a portionof the incident light not including the first portion of the incidentlight, wherein a spectral response of the photocurrent is controlledbased at least in part on a first bias voltage applied between a firstcathode contact coupled with a first cathode region of the firstphotodiode and a common anode contact coupled with a common anode regionof the first photodiode and the second photodiode.
 20. The photodetectorof claim 19, wherein a thickness of the first depletion region iscontrolled at least in part by the first bias voltage and a second biasvoltage applied between the common anode contact and a second cathodecontact coupled with a second cathode region of the second photodiode.21. The photodetector of claim 19, wherein the common anode contactcomprises: a first common anode contact coupled with the common anoderegion; and a second common anode contact coupled with the common anoderegion, wherein a resistance between the first common anode contact andthe second common anode contact depends at least in part on a thicknessof the common anode region.
 22. The photodetector of claim 19, furthercomprising: a semiconductor substrate layer comprising the firstdepletion region; and a transparent gate electrode in between a lightreception portion of the photodetector that receives the incident lightand the first depletion region, wherein the transparent gate electrodeis biased to form the first cathode region of the first photodiodebetween the first depletion region and the transparent gate electrode.